As radio access methods, Long Term Evolution (LTE) adopts uplink Single Carrier (SC)-Frequency Division Multiple Access (FDMA) and downlink Orthogonal Frequency Division multiple Access (OFDMA).
The OFDMA is a digital modulation demodulation scheme by which a plurality of carriers (sub-carriers) are multiplexed using orthogonality of frequencies. Accordingly, it is said that the OFDMA is highly resistant to fading and multipath interference. The features of the SC-FDMA are similar to those of the OFDMA. The SC-FDMA is different from the OFDMA in that carriers are continuously allocated to users. Thus, an improvement in uplink power efficiency is expected in the SC-FDMA, as compared with the OFDMA. Uplink radio resources provided in the LTE are divided into frequency and time components. The divided radio resources are allocated to users.
An LTE network configuration relevant to the present invention will be described with reference to FIG. 1. Though FIG. 1 shows an LTE network configuration according to an exemplary embodiment of the present invention, it is similar to the LTE network configuration relevant to the present invention. Therefore, the LTE network configuration relevant to the present invention will be described with reference to FIG. 1.
Referring to FIG. 1, a Mobility Management Entity (MME) 501 has a function of transmitting/receiving control signals through an S1-MME link to/from each radio base station (eNB (evolved Node B)) 503. A Serving-Gateway (S-GW) 502 has a function of transmitting/receiving user data through an S1-U link to/from each radio base station 503. The radio base stations 503 are connected through an X2 link.
The radio base station 503 can accommodate a relay base station 504. 3GPP Rel-10 specifies the relay base station 504. The relay base station 504 is connected to the radio base station 503 through a Un link, thereby configuring a cell. The radio base station 503 that accommodates the relay base station 504 is referred to as “Doner eNB (DeNB)”. The radio base station 503 serving as a DeNB has a function of transferring data between a core network (MME 501 and S-GW502) and the relay base station 504. The data transferred between the core network and the relay base station 504 is transmitted between the core network and the radio base station 503 through an S11 link. The radio base station 503 and the relay base station 504 have a function of transmitting/receiving data to/from a mobile terminal 505.
To implement load distribution in a mobile communication system having a network configuration as described above, it is necessary for the radio base station 503 to recognize the load status of the neighboring radio base station 503. As a method therefor, Non Patent Literature 1 specifies a method in which the neighboring radio base stations 503 report the load status of respective cells through the X2. Specifically, a report on the load is requested by Resource Status Request, and a response is sent by Resource Status Response. Each radio base station 503 can transmit/receive information on the usage rate of Physical Resource Block (PRB), load on HW, load on a backhaul link and load on the entire radio base station 503, by using Resource Status Update.
The handover in the LTE is executed by using as a trigger a reception quality measurement report originated from the mobile terminal 505. The types of the report at the same frequency are Events A1 to A5. Among them, Event A4 is assumed to be used in the present invention as the report. The report of Event A4 is transmitted when the reception quality of a neighboring cell, which is measured by the mobile terminal 505, exceeds a threshold. According to Non Patent Literature 2, Event A4 is transmitted when the following conditions are satisfied.Mn+Ofn+Ocn−Hys>ThreshMn: signal reception intensity of an neighboring cellOfn: offset related to a used frequency bandOcn: offset related to the neighboring cellHys: hysteresisThresh: threshold for Event A4
A handover procedure between the radio base station 503 and the mobile terminal 505 will be described below. FIG. 9 is a sequence chart showing a handover procedure in the case of using the X2 link. Referring to FIG. 9, the mobile terminal 505 measures reception qualities of the serving cell and a destination cell, and reports them to a radio base station 503-1 (step 601). The radio base station 503-1 having received the report transmits a handover request to a destination radio base station 503-2 (step 602). The destination radio base station 503-2 performs call acceptance control and notifies the result of control to the radio base station 503-1 (step 603).
The radio base station 503-1 requests the mobile terminal 505 to execute handover (step 604). The radio base station 503-1 notifies the destination radio base station 503-2 of a sequence number of packet (step 605). This prevents missing or duplication from occurring in the transmission of packets. The mobile terminal 505 notifies the destination radio base station 503-2 of completion of the handover (step 606).
The destination radio base station 503-2 requests the MME 501 to perform path switching (step 607). The MME 501 performs the path switching and sends a notification to the destination radio base station 503-2 (step 608). The destination radio base station 503-2 requests the radio base station 503-1 to delete information on the mobile terminal 505 having completed the handover (step 609).
FIG. 10 is a sequence chart showing a handover procedure in the case of using an S1 link. Referring to FIG. 10, the mobile terminal 505 measures the reception qualities of the serving cell and the destination cell, and reports them to the radio base station 503-1 (step 701). The radio base station 503-1 having received the report transmits a handover request to the MME 501 (step 702). The MME 501 transmits the handover request to the destination radio base station 503-2 (step 703).
The destination radio base station 503-2 performs a call acceptance control, and notifies the result of control to the MME 501 (step 704). The MME 501 notifies the radio base station 503-1 of the result (step 705). The radio base station 503-1 requests the mobile terminal 505 to execute handover (step 706). The radio base station 503-1 notifies the MME 501 of a sequence number of packet (step 707). This prevents missing or duplication from occurring in the transmission of packets.
The MME 501 notifies the destination radio base station 503-2 of the sequence number of packet (step 708). The mobile terminal 505 notifies the destination radio base station 503-2 of completion of the handover (step 709). The destination radio base station 503-2 notifies the MME 501 of completion of the handover (step 710). The MME 501 requests the radio base station 503-1 to delete information on the mobile terminal 505 having completed the handover (step 711).
One of methods for modifying a network configuration depending on a load is disclosed in Patent Literature 1.